This invention relates to a batch annealing apparatus for metal coils, and more particularly to a batch annealing apparatus that heats metal coils in such a manner that all portions across the width or length of the coil pass through a specific temperature range at a predetermined temperature gradient.
Conventional metal coil annealing furnaces in common use anneal a stationary coil placed on the base plate by applying a given heat pattern. In the manufacture of unidirectional electrical steels, for example, batch annealing furnaces of the type as shown in FIG. 1 are in wide use for finish annealing (secondary recrystallization annealing).
That is to say, a coil of electrical steel C is placed, with the axis thereof vertical, on a base plate 3, and an inner cover 2 is placed over the coil, as shown in FIG. 1. With a bell-shaped furnace 1 lowered over the inner cover 2, N.sub.2, AX, H.sub.2 or other atmosphere gas is supplied into the space under the inner cover 2 through a feed pipe 4. An electric heater 5 on the inside of the furnace 1 and an electric heater 6 below the base plate 3 are then turned on to simultaneously heat all parts of the coil C relatively uniformly. When the coil C has been heated up to a given temperature (1150.degree. to 1200.degree. C.) and soaked, a cooling gas is blown into the furnace 1 through a cooling gas supply pipe 7 connected to the top thereof to complete annealing at a predetermined temperature. The heated cooling gas is cooled in a cooling device 8 and recirculated into the furnace 1.
Recently, methods to reduce the size and/or weight of transformers and other electric devices using unidirectional electrical steels has become an important issue. In order to permit such size and/or weight reduction, the magnetomotive force (B.sub.8) and core loss of unidirectional electrical steels must be improved further.
The heat treatment methods the applicant proposed in Japanese Patent Applications Nos. 75,033 of 1980, 20,154 of 1981, 198,443 of 1981 and 96,740 of 1981 allows secondary recrystallization to proceed while heating coils of electrical steel at a given temperature gradient in a border region between the primary and secondary recrystallization regions. A coil of electrical steel that has undergone primary recrystallization annealing is heated from one end to the other so that secondary recrystallization is provided across the width of the coil. The heating is effected with a temperature gradient of 0.5.degree. C./cm in a border region between the primary and secondary recrystallization regions within a temperature range of 930.degree. to 1050.degree. C. These methods have made it possible to manufacture electrical steels that are unprecedentedly excellent in terms of magnetomotive force and core loss.
In the conventional batch annealing furnaces, however, the entire volume of each coil is heated substantially uniformly as described previously. Accordingly, it is impossible to provide the required temperature gradient to the coil in said border region.